as an update to the above: I ended up reverting the update and fortnite no longer drops frames. It was weird. The game played fine for 10 seconds and then I got 1fps for 3 seconds, then fine for another 10 seconds then 1fps for 3 seconds and so on. Definitely reverting the windows update fixed it though. Great. Another buggy Windows update. On their own official hardware. Again. For the twentieth time.